This one is great for the core, yet works the upper body as well. Start in a push-up position with the shoelaces of your shoes pressed against the top of the ball. Keep the belly button drawn in and your hips up high to prevent your low back from taking too much weight. With the belly drawn in slightly, start to bend your knees and bring the knees toward your chest. Do this under control. Your upper body shouldn’t move forward or sway.

Common Mistake: Letting your lower back sag as you extend your legs out. (credit: Phil Delaire)